Just a few years back (well, a coupla decades) it wasn’t well understood how planets form. There were theories, sure, and the best one of the lot was that when a cloud of gas and dust collapsed (due to a collision with another cloud, or maybe the wind from a nearby star, or getting slapped around by a supernova blast), it formed a flat disk. The star formed in the center, and the planets formed farther out.

This theory has been validated many times, and astronomers now accept it as true (I’ve written about this here, here, and here, for example). In fact, we’ve gotten pretty good at finding young stars still surrounded by their disks, and Hubble is particularly good at imaging them. The disk is faint, and can generally be seen because it reflects the light from its parent star, but that star can be thousands of times brighter than the disk. Hubble is good at high-contrast objects like that; the camera I worked on for years was awesome at imaging them (I wrote about one particular star/disk combo on my Bitesize pages).

And now, just a few years into this field of science, it’s gotten to the point where we can catalog oddities; objects that make us scratch our heads and say, "What’s going on here?"

Enter HD 15115.

HD 15115 is a young star, maybe 12 million years old, and is located about 150 light years away from us. It’s pictured at the top of this post in a new Hubble image released today (a paper with more detail was published in April). The gray circle is what’s called an occulting mask — no, not something you wear at Halloween, but a piece of metal that blocks the overwhelming light of the star so fainter objects can be seen near it. This is precisely like trying to see an airplane flying in the sky when it gets near the Sun– you hold your hand to block the Sun, and you can see the fainter airplane. The gray stripe on the left of the image is another occulting bar, called a finger.

The disk of the star is seen almost exactly edge-on. This is a fairly typical looking disk at first glance. In this false-color image, it’s bright near the star, and fades away farther out. But look again: you’d expect the disk to be symmetric around the star… in other words, the left and right sides should be about the same brightness and size.

But they’re not! On the right of the star, the disk is much brighter and longer than on the left! In fact, the left (east) side of the disk stretches out to about 47 billion kilometers from the star, while the right (west) side goes out to about 83 billion kilometers. For comparison, Pluto is about 5 billion miles from the Sun, so this is not only a huge disk, but it’s really lopsided. This result has also been confirmed using the monster 10 meter Keck telescope in Hawaii. What gives?

Well, clearly the disk isn’t symmetric. There must be more material on the right side, which makes it look brighter (it can reflect more starlight). But it’s also longer, indicating that it’s physically longer on one side than the other — meaning the disk is probably highly elliptical.

We’ve seen asymmetric disks before; usually it’s due to a planet tugging on the disk. Dust clumps up on one side, making it brighter. It’s certainly possible that’s what’s happening here.

But there is another possibility, and it’s intriguing to say the least. HD 15115 is part of a cluster of young stars that were all born at the same time. After 12 million years the cluster is breaking apart (sniff– it’s tough when they leave the nest!) and the stars are starting to scatter. Located 10 light years away from HD 15115 is another star, HIP 12545 (HD = Henry Draper catalog; HIP = Hipparcos catalog). It’s possible that HIP 12545 passed close enough to HD 15115 to disturb the dust disk, causing the asymmetry. The gravity of the passing star stretched and distorted the disk.

That’s pretty cool, if true. It’s pretty rare in general for stars to get close enough to one another to cause any real effects — the Sun can go billions of years without it happening — but the odds get stacked when the stars form in clusters. Encounters are more common, and so it’s possible a stellar interloper whacked up the disk around HD 15115.

I expect we’ll see more of these as we get better at observing young stars. I also expect we’ll get more surprises, too. One of the most fun parts about a young field of science like this one is that just as you start to get comfortable and think you understand what’s happening, bam! The Universe tosses something like this at you and you have to rethink your ideas. It distorts your disk, so to speak.

But it also keeps you on your toes. Scientists who study objects like this need to be ready to reshape their ideas when they get a curveball, and it’s one of the reasons science is so strong. Mental exercise and science: they’re good for your brain.

Comments (16)

So why doesn’t the disk even out over time (unless there is planet there). I’d have thought that after a number of orbits, it would even out – the stuff near the star (on whichever side) would overtake the stuff on the other side (that’s further out) just because a closer orbit rotates around the star more quickly. But it’s lopsided from the middle all the way out. Weird.
Unless the disk’s own gravity gets involved. Beyond me.

First I thought it was something like a solar flare, but it’s cool enaugh!

Even out? Well I think, once you disturbed that thing it will take A LOT (if not “for ever”) of time to make a nice round-shaped disk again. I’m thinking here about “entropy”, a round-shaped disk is very special I guess, so it’s unlikely that it will go back to this state.

Another point, Phil mentioned that the star has built inside a cluster of stars and they are moving out now… maybe the star is rushing to the “left” in ths picture

The most wonderful feeling in the Universe is “BAM!” (And closley related to one of my favorite quotes by Dr. Asimov, which goes something like: “The really exciting phrase in Science, the one that heralds a new discovery is not “Eureka!”, but rather, “Hmmm, that’s funny”.”

The prime difference, IMHO, between Scientists and *ahem* others, is that Scientists are not only not afraid of the BAM, but actually seek it out. In a sense, a real Scientist is a BAM-o-maniac…

Perhaps that’s why those of us who are fans of Science, but do not possess advanced degrees, or professions in which we can directly experience BAM come on here: We get our daily BAM fix, and that helps us get through the ‘jones’ that the world inflicts on us (As evidence, try, just try going without your daily dose of BA, or APOD, or Skepchick, or any of the other excellent Science websites for a week. Then, when you are channel surfing, FORCE yourself to flip PAST the Discovery channel, the Science channel, TLC, and even Animal Planet, but be sure to have a ready supply of your favorite calming potion handy…)

If Hypothetically, the sun had developed in such an open cluster (it is possible, considering the ~4.6 billion years of formation and subsequent dispersal), and another star had perturbed the initial solar planetary disk, would there be evidence in our solar system? Wouldn’t the perturbed disk eventually reach equilibrium, sans some mass from the ejected material?

How can we rule out the possibility that such an event didn’t occur to our planetary system?

Is it possible that billions of years from now, this star HD15115, would develop into a planetary system without any evidence of its archaic encounter with HIP12545?

– If the near miss with HIP 12545 caused such an asymmetry for HD15115, did the close encounter leave any telltale traces on HIP12545?

– The Keck article mentions the missing mass and the lower dust level of this Blue Needle compared to its group stars nearby. If, by any chance, this points to some planets already formed around HD15115, what is the minimum time required to form planets? Somehow 12 million years sounds extremely young.

Aren’t orbits eliiptical with the center of mass at one of the foci? It might appear that this planetary system is developing with rather more eccentric orbits than most. Yes, I’d expect, after the bodies coalesce, the orbits would precess out of all order, but would Saturn’s ring system be as stable if Saturn were at one focus of a family of very eccentric ellipses?

Helioprogenus, I wouldn’t think that orbital eccentricities of that magnitude would be reduced just by gravitational forces acting between developing planets within the disk. I would think that the orbits would remain fairly eccentric unless the developing disk has another close encounter with a star sufficient to perturb the orbits within the disk a second time.

I think that question would be better answered by the BA, though, as I am a toxicologist, not an astronomer.

Elliptical orbits are just as stable as circular ones, so if the disk particles didn’t interact the elliptical disk could be stable forever. The individual particles would certainly get out of sync – they already are – just as they do in a circular system, because the inner ones orbit faster than the outer ones. But if they’re all orbiting in ellipses, and the axes of those ellipses are all aligned, then the elliptical outline won’t change.

Of course, the particles do interact. They collide, they have gravitational interactions, the bulk mass of the disk pulls them around, and probably fifteen other effects I haven’t thought of. Any such interaction is going to change the orbits of the objects involved. It probably won’t circularize them directly, but at the least the eccentricity and particularly the orientation of the ellipse they follow will change. If you imagine it as gradually randomizing the orientations, then after some amount of time you’ll have a lot of particles moving in ellipses, but the eliipses will be pointing every which way. If they get to the point that they’re evenly distributed, so that the typical size in every direction is the same, then you have a circular disk, even though all the orbits might be elliptical!

The orbits probably won’t stay elliptical, since elliptical orbits in an arrangement like that will be constantly crossing each other, causing lots of interactions; the system will tend to circularize (since circular orbits don’t interact so much, it’s a more stable configuration) and we’ll be back to a normal disk situation.

The interactions that happen in a circular disk situation are what allow material to migrate inward and ultimately fall into the central object. When the interactions fall off, because there just aren’t that many objects left, you’re left with a very long-lived disk or ring, like Saturn’s; it may be so sparse it’s a planetary system.

What I find really interesting about this one is that it must have gotten a kick – a particular, well-ordered kick – pretty recently, so it hasn’t had time to circularize itself. I wonder how long that process takes? I don’t even have an order-of-magnitude idea of the timescale. The shorter it is, the harder it is to believe some rare event caused this (because there’s less time for it to have happened in). So I bet the disk modelling people are having a field day with this one (and maybe a few shouted arguments with their cluster dynamics colleagues). Anyone have a number for the circularization time?

I believe there are examples of three-armed spirals out there, am I correct? If so, then a view “plane-wise” would exhibit the same dis-proportionate mass dispersion, with the possible exception if we were looking at the third arm while it was on the other side of the spiral, but centrally located. Though, of course, it may give the same result if it were on our side.

If it was simmetrical, it would not have attracted as much attention, but being lop-sided, we have to account for it. So if we have parts of two arms on one side, from our perspective, then we can expect more mass and/or greater extension in the view. By the way, one could see the same effect looking at a propellor or even a fan – three-bladed types of course.

Have we had enough time to get a fix on any differential movement of stars or mass of gas in those spirals?

Looks as though we have the “regex” thing happening again as we, or at least, I, try to submit a comment. I tried to submit again, but at least the system picked that up and so didn’t double the submission.

Also I am currently engaged in writing out a hundred times, “symmetrical”. I knew there was something wrong with my first and second attempts at spelling it, but I forgot to look it up.